The natural maintenance cycle of La Crosse encephalitis arbovirus will be continued in hardwood deciduous forested study areas in the large endemic area in rural western Wisconsin and in foci of virus activity as recently found in suburban eastern Wisconsin. Endemic and non-endemic areas of La Crosse virus activity will be further defined by following the distribution and exposure histories of cases of California encephalitis, antibodies to La Crosse and other California group arboviruses and by isolates of La Crosse virus. Maintenance mechanisms including transovarial, venereal, and oral transmission will be studied using field strains of La Crosse virus and Aedes triseriatus mosquitoes collected from endemic and non-endemic areas. Ten-fold increases in venereal infection and subsequent oral transmission rates recently observed in Aedes triseriatus females infected following previous engorgements on chipmunks will be further delineated. Reductions of venereal infection and transmission rates in females mated after previous engorgements on chipmunks with antibody will be studied as a possible natural control mechanism for La Crosse virus.